Review of the hydraulic capacity of some existing culverts on selected roads in Kumasi Metropolis (Ghana)

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Culverts are one of the most important and common crossing structures that allow roadways to traverse rivers and streams. Due to their importance, their design and construction are to be done in such a way as to avoid overflow of water. Inadequate sizing of culverts leads to ponding of water behind the structure and eventually flooding of the culvert and the roadway. Inadequate sizing of culverts in the country has come about because engineers have been relying on their experience to select the sizes without going through vigorous computational design. This research work looks at 16 culverts in the Kumasi metropolis, which have been experiencing frequent flooding. The 16 culverts were selected after series of interviews, consultation and field visits with the various road agencies of the Kumasi metropolis. Thereafter, landsat images for various years were used to obtain catchment areas contributing flow to the various culverts. The satellite images also enabled us to determine areas of vegetation and settlement over the years. Areas of vegetation represent areas of high permeability while areas of settlement denote areas of high imperviousness. With these areas of settlement and vegetation, runoff coefficients of the contributing catchment were calculated over the years. Times of concentrations were then computed by the Brandy Williams equation. The intensities of rainfall were then defined at 25 years return period with the aid of IDF curves of Kumasi and subsequently the discharge at the various culvert sites computed using the modified rational formula. The Civil 3D software was then used to size the various drains at different years. The results show that out of the 16 culverts investigated only four (4) were found to have adequate sizes and that the frequent floods that were experienced here were due to other factors such as silting and blockages while twelve (12) were found to be inadequate. The results show that in general some form of computation, especially determination of hydrological flow at the culvert site must be done to enable quick use of culvert nomograms for selecting the appropriate culvert size.
A thesis submitted to the Department of Civil Engineering, College of Engineering in partial fulfilment of the requirements for the degree of Master of Science Roads and Transportation Engineering,